1. Field of Invention
The present invention relates to a package structure having tapering support bars and leads. More particularly, the present invention relates to a type of tapering support bar and lead used in the common lead frame of a quad flat non-leaded package.
2. Description of Related Art
Methods of manufacturing integrated circuits have developed quite rapidly. Besides the miniaturization of semiconductor devices, compact packages also have been developed. Nowadays, chip scale package (CSP) is one of the most common types of structures for housing a semiconductor chip. A chip scale package is only slightly larger than the semiconductor chip so that its volume occupation is relatively small and its signaling routes are relatively short. The carrier inside a chip scale package includes a lead frame, a flexible substrate and a rigid substrate. Because material cost of lead frames is low and easy to process, most chip scale packages use a lead frame. Quad flat non-leaded package (QFN) is one such lead frame based chip scale package. In general, a leadless chip scale package is used to encapsulate a low pin count semiconductor chip due to its short signaling route and small signal attenuation.
FIG. 1 is a schematic cross-sectional view of a conventional quad flat non-leaded package. FIG. 2 is a bottom view of the quad flat non-leaded package shown in FIG. 1.
As shown in FIGS. 1 and 2, structure and technique for fabricating a quad flat non-leaded package (QFN) is disclosed in U.S. Pat. No. 5,942,794 (Matsushita, 1999). The QFN structure is built around a lead frame. The lead frame has a die pad 100 in the middle surrounded by a plurality of leads 102. The QFN structure also includes a die 104 having an active surface 106 and a backside 108. The active surface 106 has a plurality of bonding pads 110 thereon serving as connection points from the die 104 to devices outside the package. The backside 108 of the die 104 is bonded to the die pad 100 using an adhesive material 112. The bonding pads 110 are electrically connected to the leads 102 using conductive wires 114. In general, the die 104, the die pad 100, the conductive wire 114 and the upper surface 118a of the leads 102 are enclosed by plastic material 116. Therefore, only the lower surface 118b and side surface 118c of the leads are exposed as external contacts.
In the fabrication of a leadless package, the interconnected packages must be separated into individual packages in a singulation process after the lead frame is encapsulated by a plastic material. However, sudden stress on the leads 102 or support bar section hardly can be avoided regardless if the singulation is conducted by sawing or punching. This type of impact often leads to a slight parting between the lead or support bar and the packaging material and results in delamination. Hence, moisture sensitivity level (MSL) of the product will be decreased, and reliability of the product is compromised.
Accordingly, one object of the present invention is to provide a lead frame having tapering support bars and leads capable of reducing delamination of the plastic material in an integrated circuit package and hence increasing reliability of the package.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a package having tapering support bars and leads. The package includes at least a lead frame, a die, a plurality of conductive wires and a plastic housing. The lead frame has a first surface and includes at least a package unit. The package unit includes a die pad, a plurality of leads and a plurality of support bars. The die pad is positioned in the middle. The leads and support bars are distributed around the periphery of the package unit. In addition, the width of the leads and support bars decreases gradually from a location close to the die pad towards the peripheral region. The leads and support bars have a rectangular or trapezoidal cross-section. A die is bonded on the surface of the die pad, and the die is electrically connected to the leads on the lead frame via a plurality of conductive wires. Plastic material such as epoxy resin encloses the die, the conductive wires and the first surface of the lead frame.
The gradual reduction of the width of the leads and support bars from a region close to the die pad towards the peripheral region of the package unit can reduce some of the pulling stress in a punch-shearing operation. Consequently, bonding strength between the leads and support bars on one hand and the encapsulating material on another is increased, and delamination of the plastic body of the package is prevented.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.